In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected. The sheet variables that are most often measured include basis weight, moisture content, and caliper, i.e., thickness, of the sheets at various stages in the manufacturing process. These process variables are typically controlled by adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, and/or varying the nip pressure between calendaring rollers at the end of the process. Papermaking devices are well known in the art and are described, for example, in “Handbook for Pulp & Paper Technologists” 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc. Sheetmaking systems are further described, for example, in U.S. Pat. No. 5,853,543 “Method for Monitoring and Controlling Water content in Paper Stock in a Paper Making Machine,” U.S. Pat. No. 5,891,306 “Electromagnetic Field Perturbation Sensor and Methods for Measuring Water Contents in Sheetmaking Systems,” and U.S. Pat. No. 6,080,278 “Fast CD and MD Control in a Sheetmaking Machine,” which are all assigned to the common assignee of the instant application.
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (wet stock) on a traveling mesh wire or fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The papermaking machine is essentially a de-watering, i.e., water removal, system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
Conventional methods for controlling the quality, e.g., basis weight, of the paper produced include regulating the paper stock, e.g., chemical composition and/or quantity, at the wet end of the papermaking machine. For example, the thickness of the paper at the dry end can be monitored to control the flow rate of wet stock that goes through valves of a headbox and onto the mesh wire.
In order to precisely measure some of the paper's characteristics, it is essential that the fast moving web of paper be stabilized at the point of measurement to present a consistent, flat profile since the accuracy of many measurement techniques requires that the web stay within certain limits of flatness, height variation and flutter. Moreover, to avoid paper degradation, stabilization must be accomplished without contact to the stabilizing device. This is critical at the high speeds which web material such as paper is manufactured.
Current non-contact sheet stabilizers fall into two general categories on the basis of their characteristic operation. The first category includes various air clamps that use only airflow to impart some degree of suction on the web material to urge the web material against a flat surface of the device. These air clamps have a tendency to leave marks or otherwise damage the moving web. The second category includes air clamps that use airflow to impart suction but that also generate an air bearing between a surface on the device and the web material. The latter category of stabilizers is exemplified by Vortex, Coanda and Bernoulli-type air clamps which cushion the moving web material with an air bearing as the web travels over the device. Vortex-type air clamps provide adequate air bearing support but create a “sombrero-type” profile on the web material in the center of its effective region, thus they do not generate a sufficiently flat profile. Bernoulli-type air clamps, which blow air out of recessed openings horizontally over a surface, cause the web material to contact the surface and flutter. Finally, simple Coanda slot-type air clamps provide an air bearing and a flat profile adjacent the Coanda slot but lack the ability of retaining sufficient sheet flatness along the flow direction away from the Coanda slot. The Coanda effect is a phenomenon whereby a high velocity jet of liquid issuing from a narrow slot will adhere to a surface it is traversing and will follow the contour of the surface.
As is apparent, the art is in need of a non-contact air clamp stabilizer for fast moving web materials that is able to present a flat profile of the web for analysis and that is robust in response to changes in web (machine) speed and/or weight.